An actuator, which is a device for triggering various mechanical movements, is utilized in various fields from various industrial machinery, automobiles, aircrafts, medical equipment, to personal electric products.
Among these, a piezoelectric actuator that converts electric energy to mechanical energy utilizing the piezoelectric effect or the electrostrictive effect is widely used. Characteristics of the piezoelectric actuator are that conversion efficiency from electric energy to mechanical energy is high, it is small, control of minute displacement is easy, and an electric current is not effectively flowing when a voltage is applied so as to maintain a state of a given generated displacement, namely there is power consumption while the applied voltage is reached, yet there is no further power consumption when maintaining a uniform voltage thereafter. A constituent material of such a piezoelectric actuator may be crystal, a ceramic material, an organic polymer material such as polyvinylidene fluoride (PVDF) and the like.
FIG. 5 illustrates an exemplary piezoelectric actuator configured as a clamping mechanism. The clamping mechanism is structured so as to convert generated displacement of a multilayer piezoelectric actuator to mechanical movement of pinching an object. FIGS. 5(a) shows a state where a voltage is not applied to the multilayer piezoelectric actuator, and 5(b) shows a state of pinching a target object by applying a voltage to the multilayer piezoelectric actuator.
In the case of using such a piezoelectric actuator, detection of the state of force generated at the tip of the piezoelectric actuator is necessary. For example, in the case of configuring a clamping mechanism for a chip component mounting device, it is necessary to surely pick up with a force that overcomes the inertial force imparted on a chip component due to acceleration when moving the chip component. Moreover, in the case of pinching and moving a breakable object such as a wine glass or an egg, it is necessary to adjust the applied voltage so as to provide an appropriate force within a range of force, which can not break the target object even when the wine glass or the egg is moving where the inertial force is additionally imparted thereon. Furthermore, since generally there are fluctuations in dimensions of the target object, it needs to be structured so as to exert a predetermined force irrespective of degree of generated displacement of the piezoelectric actuator.
In order to exert such an appropriate force within the realm of conventional technology, it is necessary to use a force sensor that detects reactive force from the target object at the tip of the clamping mechanism, and adjust the applied voltage imparted on the piezoelectric actuator so that the value of the force sensor is a desired value. However, since a mechanical sensor such as a typical force sensor is expensive, and the force sensor must be placed between the clamping mechanism and the target object in order to incorporate the force sensor, which detects external force, into the clamping mechanism, the execution itself is often difficult.
The above problem may be resolved if estimation of generated force and generated displacement according to measured values of voltage and electrical quantity on the input side of the piezoelectric actuator can be substituted for incorporation of a displacement sensor or a force sensor. With this kind of objective, research of self-sensing actuation (denoted as SSA hereafter), which aims to simultaneously carry out sensing quantity of displacement and then carry out actuation accordingly, is being conducted (e.g., Non-Patent Document 1). SSA attempts estimation of generated displacement from voltage applied to the actuator, and strives for reduction in size and weight as well as low cost of a control system by sensing the quantity of displacement from the voltage without using a displacement sensor. SSA allows estimation of generated displacement in a state where there is no mechanical load, and estimation of force to be generated if a certain mechanical load is estimated. However, since only voltage is measured, quantity of two independent mechanical systems such as displacement and force is impossible. For example, according to SSA technology, if there are fluctuations in dimensions of the target object in the case of the aforementioned clamping mechanism, error in estimated generated force occurs.